Thin film resistor fabricated on header

ABSTRACT

An igniter assembly for a gas generator used to inflate air bags or activate seat belt tensioners includes a header having a bridge element fabricated on a surface of the header. A pyrotechnic charge is applied to the bridge element so that delivering energy to the header heats the bridge thereby igniting the pyrotechnic material. The header includes metal sections separated by a insulating section of low thermal conductivity. A resistive film is applied to the header, across the metal sections and the insulating section. The bridge element is formed by laser cutting a kerf into the resistive film and thereby directing all current between the metal sections through a section of the film which comprises a resistor.

[0001] This invention relates to a thin film bridge element for an igniter to actuate a gas generator of the type used to inflate air bags and activate seat belt tensioners.

BACKGROUND OF THE INVENTION

[0002] Current generation air bag and seat belt tensioners use a pyrotechnic material such as lead styphnate in contact with a bridge element to ignite a gas generator. The gas produced by the generator inflates an air bag or retracts a seat belt to provide the desired tension. The bridge element is typically a thin film resistor of tantalum nitride, nichrome, or other suitable material of a size determined by the availability of energy and other thermal characteristics of the film and substrate. The pyrotechnic material placed adjacent to the thin film bridge is referred to as the abutting charge and may be a single material or a multipart pyrotechnic charge. In some designs, the pyrotechnic charge is compacted against the thin film element at substantial pressures to achieve intimate thermal contact between the thin film bridge and the abutting charge.

[0003] The most common form of thin film bridge is a custom thin film chip resistor comprised of a thin film element of predetermined size which terminates in appropriately low resistance metals to connect to a source of voltage. The thin film chip resistor is soldered to the header, having one terminal in electrical contact with a central pin and the other terminal in electrical contact with a metal sleeve. Applying a voltage across the pin and metal sleeve delivers current through the resistor thereby heating the resistor and the abutting pyrotechnic charge. When the temperature of the pyrotechnic charge reaches an ignition value, the pyrotechnic charge ignites thereby setting off a chain of events resulting in the inflation of an air bag or tensioning of a seat belt.

[0004] The chip resistor is mechanically and electrically connected to the inside of a plastic or metal housing containing a metal header which also contains the ignition charge as shown in FIG. 1. In this conventional design, there are several distinct disadvantages. The first is the need of forming electrical connections between the thin film chip resistor using wire bonding, soldering, or conductive adhesive techniques. The second is the potential for damaging the element during high pressure compaction of the abutting charge to the bridge. Microcracks formed during high pressure application of the pyrotechnic charge could lead to failure during the life of the vehicle and prevent proper deployment of the air bag or prevent operation of the seat belt tensioners.

[0005] Disclosures of some interest relative to this invention as found in U.S. Pat. Nos. 4,525,238; 4,831,933; 5,798,476 and 5,939,660.

SUMMARY OF THE INVENTION

[0006] In this invention, advantage is taken of the design and construction of the header to which the bridge element and pyrotechnic material are attached. Headers provide a mechanical and electrical connection between the controllers of air bag and/or seat belt systems and the gas generator that inflates them. Headers comprise two metallic conductors separated by an insulating body. In one embodiment of the current generation of air bag and seat belt tensioning systems, the headers are of cylindrical shape having a central pin providing one of the electrical connections, an insulating sleeve around the pin and an annular metal body, around the insulating sleeve, providing the other electrical connection. The upper surface of current generation headers are flat, typically having been abraded to a flat surface.

[0007] In this invention, the thin film element or bridge is fabricated directly on the insulating sleeve of the header by depositing a thin film resistive material on substantially the entire flat surface of the header and then laser cutting a path or kerf through the thin film material on the insulating sleeve. The laser cut path electrically separates the central pin from the annular metal body except for a small area of the thin film material which acts as the thin film bridge or igniter element. In this manner, all current applied to the electrical terminals of the header must pass through a small predetermined area, on the insulating sleeve, of the thin film resistive material.

[0008] In this manner, a bridge element is fabricated directly on a surface of the header so that no subsequent manufacturing operations are necessary to provide an electrical connection between the bridge element and the electrical components of the system. In other words, whatever steps are taken to connect the header to its abutting components also provides the electrical connection to the bridge. It is thus apparent that savings are achieved through a reduction in assembly time, effort and equipment and the reduction in materials making up a conventional thin film chip resistor. Simultaneously, reliability and yield are improved by the elimination of defectively assembled chip resistor type bridge elements. In addition, reliability and yield are improved by making the bridge more robust and capable of withstanding greater shocks and higher pressures, such as used to apply pyrotechnic charges to the header.

[0009] An interesting feature of this invention is the sensitivity or efficiency of the bridge element. The requirement of the bridge element is to provide a predictable rise in temperature upon the application of a predetermined amount of energy to the device. In this invention, there is a substantially improved temperature response because of the nature of the underlying insulating substrate on which the bridge element is fabricated.

[0010] It is accordingly an object of this invention to provide an improved thin film bridge which is deposited directly onto an insulating portion of a gas generator header.

[0011] Another object of this invention is to provide a thin film bridge of improved reliability due to the elimination of conventional electrical and mechanical connections.

[0012] A further object of this invention is to provide a thin film bridge which is not damaged by high pressure application of a abutting pyrotechnic charge.

[0013] Another object of this invention is to provide a thin film bridge which produces a large temperature response by the application of a small amount of energy.

[0014] These and other objects and advantages of this invention will become more apparent as this description proceeds, reference being made to the accompanying drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is an isometric view of a prior art header and bridge combination;

[0016]FIG. 2 is an isometric view of a conventional header having the bridge of this invention fabricated thereon;

[0017]FIG. 3 is an enlarged top view of part of the device of FIG. 2;

[0018]FIG. 4 is an enlarged cross-sectional view of the device of FIG. 3, taken substantially along line 4—4 thereof, as viewed in the direction indicated by the arrows;

[0019]FIG. 5 is a schematic view of a circuit in which the device of this invention is used;

[0020]FIG. 6 is a graph showing the thermal response of the bridge element of this invention;

[0021]FIG. 7 is an isometric view of another embodiment of this invention; and

[0022]FIG. 8 is a top view of a jig or array of headers ready to be positioned adjacent a laser cutter.

DETAILED DESCRIPTION

[0023] Referring to FIG. 1, a prior art header-bridge assembly 10 comprises a conventional header 12 and a bridge element in the form of a thin film chip resistor 14 connected, as by soldering or the like, to the top of the header 12. A pyrotechnic charge 16 is applied to the top of the resistor 14 in any suitable manner, as by compaction of a finely powdered material or by application of a liquid slurry of the material. Conventional headers are of several types. As shown in FIG. 1, the header 12 comprises a central metal pin 18 surrounded by an insulating sleeve 20 which, in turn, is surrounded by an annular metal sleeve 22. The pin 18, insulating sleeve 20 and metal sleeve 22 are bonded together and the upper surface 24 is ground flat in a conventional manner. The insulating sleeve 20 is designed to electrically insulate the pin 18 from the sleeve 22.

[0024] A second pin 26 connects to the sleeve 22 and both pins 18, 26 extend below the bottom of the sleeve 22 for connection to a control device. It will accordingly be seen that the pins 18, 26 provide a convenient means of electrically connecting the header 12 to the remainder of a circuit shown in FIG. 5, as will be more fully apparent hereinafter. To actuate an air bag or seat belt tensioner, a current is delivered through the pins 18, 26 thereby heating the chip resistor 14, igniting the pyrotechnic charge 16 and setting off the gas generator (not shown). The headers 12 are of conventional design and are available commercially from Shott Glass Technologies of Duryea, Pa. 18642.

[0025] The thin film chip resistors 14 are produced in large quantities and are attached, mechanically and electrically, by soldering the resistor 14 to the header 12. This approach has the disadvantage of possible electrical or mechanical failure between the bridge 14 and the header 12 during storage over the life of the vehicle in which they are incorporated. Failures can occur due to thermal cycling of the components and result in the loss of electrical connection following solder joint failure or the failure of wire bonds. The frequency and severity of such failures can be increased by the high pressure application of the pyrotechnic charge 16. The effect of failures of this type manifestly depends on when the failure occurs. In this invention, external electrical and mechanical connections between the bridge element 14 and the header 12 do not rely on solder or wire bond joints so there is no possibility of them failing.

[0026] Referring to FIG. 2, a bridge-header assembly 28 of this invention is applied to a conventional header 30 having a central pin 32, an insulating sleeve 34 and a metal sleeve 36 providing an electrically connecting pin 38. Fortuitously, for purposes of this invention, the sleeve 34 is conventionally made of a thermally insulating material, and is typically glass. Glass has a low thermal conductance and typically is in the range of 0.01-0.03 watts/centimeter-K. Low thermal conductance of the sleeve 34 is an important aspect of this invention because it limits the transfer of heat to the high thermal conductance metal pin 32 and metal sleeve 36. In this invention, the sleeve 34 has a thermal conductance of .04 watts/centimeter-K or less.

[0027] During the process of grinding the top 40 of the header 30, the insulating sleeve 34 becomes slightly depressed relative to the top of the pin 32 and sleeve 36, as shown in FIG. 4. This depression is exaggerated in FIG. 4 and, in production versions of the header 30, this depression is small, hard to see but can be detected with stylus measuring equipment.

[0028] In this invention, a thin film layer 42 of resistive material such as tantalum nitride, chromium, nichrome, hafnium diboride, hafnium nitride or the like, is applied to all, or substantially all, of the flat surface 40 of the header 30 in a conventional manner, as by sputtering, vacuum evaporation or the like. The thickness of the thin film layer 42 is an important part of the geometry of the resistor that is ultimately fabricated on the top 40 of the header 30, as is discussed more fully hereinafter.

[0029] As also more fully discussed hereinafter, a conventional laser machining device, such as is commercially available from GSI Lumonics, Inc. of Bedford, Mass., is used to cut a kerf or slot 44 in the thin film layer 42 above the insulating sleeve 34 to divide the thin film layer 42 into three regions 46, 48, 50. The region 46 is electrically connected to the central pin 32 and is electrically isolated from the sleeve 36 except through the region 48. The region 50 is electrically connected to the metal sleeve 36 and thus the pin 38 and is electrically isolated from the pin 32 except through the region 48. The region 48 is accordingly a resistor through which all current passing between the pin 32 and sleeve 36 must flow.

[0030] The kerf or slot 44 is accordingly wide enough to insure that the regions 46, 50 do not communicate or conduct current except through the region 48. A conventional laser machining device cuts a slot about 15 microns wide which is sufficient for most practical purposes. In the event a wider slot is necessary or desirable, the laser machining device can be defocused slightly to provide wider kerfs, a second kerf can be cut slightly offset, but overlapping, the kerf 44, or the like.

[0031] Because the sleeve 34 is annular, the kerf 44 includes a large segment 52 which is essentially circular but which may be of any desired configuration provided that it effectively isolates the region 46 from the region 50 so that all current must flow through the region 48. The movement of the laser machining device is typically by way of an x-y positioner (not shown) so the segment 52 appears to be of a smooth circular shape. On magnification, however, the kerf segment 52 is shown to be made of short, relatively straight sections. The region 48 is preferably but not necessarily rectilinear, i.e. square or rectangular, because resistors are conventionally rectangular so that design calculations are simplified. To this end, the kerf 44 includes a pair of parallel segments 54 defining the sides of the resistor region 48 and end segments 56, 58 which intersect and are generally perpendicular to the segments 54. This causes the resistor region 48 to act as if it were a rectilinear resistor and design calculations match well with actual performance.

[0032] It will accordingly be seen that the regions 46, 50 are analogous to the conductive leads of the chip resistor 14 and the region 48 is analogous to the resistive element of the chip resistor 14. The regions 46, 48, 50 accordingly constitute a bridge or bridge element for igniting a pyrotechnic charge.

[0033] Referring to FIG. 5, a circuit 60 provides a source of energy 62, which is typically a small capacitor, a switch 64, the header 30 and a resistance 66 which is the resistance of the circuit 60 and is known as the source resistance which includes the effective series resistance of the capacitor 62, the resistance of the switch 64 and the resistance of other interconnections. The switch 64 is typically a semiconductor which has an effect on the power curve of FIG. 6 as will be more fully apparent hereinafter. The header 30 provides the resistor 48 but also includes a resistor 68 which is the resistance of the region 46 and a resistor 70 which is the resistance of the region 50. When the switch 64 closes, the source of energy 62 connects to the resistors 48, 68, 70 and the resistor 48 heats up to a temperature sufficient to ignite the pyrotechnic charge 72, applied or bonded to the resistor 48, thereby setting in motion a series of events to inflate an air bag or actuate a seat belt tensioner.

[0034] Referring to FIG. 6, a curve 74 shows the power supplied to the chip resistor 14 having a conventional alumina substrate or to a resistor 48 of this invention providing 5 ohms of resistance assuming a 7,000 square micron resistor, a source resistance of 5 ohms and a capacitor of 1 microfarad capacity and having a potential of 24 volts. Because the switch 64 is of a semiconductor type, it does not close immediately so the peak energy supplied to the resistors 14, 48 does not peak for a few microseconds. The curve 76 shows the temperature response of the conventional chip resistor 14, providing a peak ignition temperature rise of about 500 K so that the ultimate temperature is about 500 K above ambient which is sufficient to ignite the pyrotechnic charge 16. The curve 78 shows the temperature response of the resistor 48 of this invention providing a peak temperature rise of about 1000 K. This illustrates that the energy efficiency of this invention is substantially better than the prior art. The reason is the insulating sleeve 34, which is normally glass, has much lower thermal conductance than the substrate of the chip resistor 14 which is normally alumina. Thus, in this invention there is very little heat lost from the resistor 48 through its substrate 34 in the few microseconds that is necessary for ignition of the pyrotechnic material to occur. Alumina, on the other hand, has a surprising high thermal conductance and acts as a heat sink. This increase in efficiency allows one of three design alternatives: use smaller capacitors, use resistors of larger area which makes them less responsive to small errors of manufacture and thereby improves their reliability, or some combination thereof.

[0035] Referring to FIG. 7, another embodiment of this invention comprises a bridge-header assembly 80 comprising a header 82 of slightly different configuration having a pair of parallel pins 84, 86 offset to but symmetrical relative to a central axis 88. An insulating sleeve 90, typically of glass, surrounds the pins 84, 86 and insulates them from a annular sleeve 92. The header 82 is abraded so the pins 84, 86, sleeve 90 and sleeve 92 have a flat upper surface 94. Those skilled in the art will recognize the header 82 as being of conventional design.

[0036] In accordance with this invention, a thin film or layer 96 of resistive material of any suitable type is applied to substantially all of the flat upper surface 94 in any suitable manner, as by sputtering, or vacuum deposition or the like. Using a laser machining device, a kerf 98 is cut in the layer 96 down to the upper surface 94 of the insulating sleeve 90 to provide a region 100 in communication with the pin 84 and electrically isolated from the pin 86 and the sleeve 92 except through a region 102 and a region 104 in communication with the pin 86 and electrically isolated from the pin 84 except through the region 102. Conveniently, this is accomplished by a first generally circular kerf 106 through the layer 96 that electrically isolates the pins 84, 86 from the sleeve 92 and a pair of second kerfs 108 that restrict current flow through the region 102. The second kerfs 108 intersect the kerf 106 and thereby compel current flow through the region 102. The ends 110 of the second kerfs 108 are parallel thereby providing a rectilinear, by which is meant square or rectangular, shape for the region 102. It will accordingly be seen that the region 102 comprises a bridge for igniting a pyrotechnic charge 114 abutting or bonded to the region 102.

[0037] Referring to FIG. 8, there is illustrated a jig or tray 116 having a series of recesses 118 therein sized and shaped to receive the headers 30, 82. The tray 116 is placed in a laser machining device of a conventional type so the laser thereof cuts the kerfs 44, 98 on top of the sleeves 34, 90 as shown in FIGS. 2-4 and 7. One advantage of this invention is that the tray recesses 118 do not have to be positioned exactly relative to the laser of the laser machining device. The only requirement is that the kerfs 44, 98 have to be on the insulating sleeves 34, 90 so the tray recesses can be off a few mils relative to the laser. As will be more fully pointed out hereinafter, the kerfs 44, 98 have to be accurately cut but there is some tolerance where the cuts begin.

[0038] During the manufacturing process, it is important to achieve close tolerance on the resistor so the desired thermal response is achieved. The typical requirement for an element with an area of 10,000 square microns is plus-or-minus 10% in area. This means the length and width of the resistor are within plus-or-minus 5% on dimensions in the range of 100 microns. This can be accomplished by standard photolithographic techniques but this approach is not practical for elements deposited on individual headers. In this invention, no photolithography or etching is required or used. Laser machining properly dimensions the resistor element.

[0039] The resistive thin film layers 42, 96 are deposited in a conventional manner, as by placing the headers 30, 82 on pallets which are traversed across the deposition plane of a conventional vapor deposition system which is typically a sputtering system, evaporation by resistance heating or electron beam. The deposition occurs by directing the vapor stream up or down depending on the choice of fixtures to position the headers. The thickness of the thin film layers 42, 96 is selected from the equation:

t=ρ/Rs

[0040] where ρ is the specific resistance of the deposited material and Rs is the desired sheet resistance and is determined from the equation:

Rs=Ro/n

[0041] where Ro is the desired ohmic value of the element and n is the aspect ratio of the bridge element, i.e. length/width. The sheet resistance Rs of thin film resistors is typically in the range of 0.5-5 ohms/square and the aspect ratio is typically about 0.5-10. A typical thickness of the thin film layers 42, 96 of this invention is on the order of 0.1-3 microns.

[0042] After the resistance material is applied to the headers 30, 82, the headers are placed in the trays 116 and taken to the laser machining device. Many commercially available lasers have sufficient power and positional accuracy to be suitable for use in this invention. Although the YAG laser is most common, excimer, CO₂ and other lasers are suitable.

[0043] It will be realized by those skilled in the art that the total final resistance of the bridge-header assembly 28 after laser machining will be comprised of the resistors 48, 68, 70, i.e. the resistances of the regions 46, 48, 50. The value of the resistor 48 is Re=Rs * Le/We. The value of the resistors 68, 70 are each:

Rc=Lc/Wc+ΔN

[0044] where Rc is the resistance of one of the connecting regions 46, 50, Lc is the length of the same region 46, 50 as measured from the pin or housing to the end of the element (which element, i.e. put a number in here) and Δ N is a transition resistance with occurs due to the abrupt change in path width, which exists where the connecting regions 46, 50 join the resistive region 48. For simplicity, the width of the connecting regions 46, 50 are taken to be twice the length of the connecting regions 46, 50.

[0045] Because the dimension of the regions 46, 50 are fixed, their effect can be calculated. In the typical dimensions of this invention, the total effect of the regions 46, 50 are 2.66 squares of film while the region 48 is 3 squares of film. For a sheet resistance of 0.88 ohms/square, the resistances of the regions 46, 48, 50 are 1.17, 2.66, and 1.17 ohms respectively. Thus, approximately 53% of the energy from the source 62 is available for joule heating of the resistive region 48, excluding losses due to capacitor effective series resistance, switch resistance and interconnection resistance.

[0046] This would appear to be a large loss but does not, in fact, create any problem even when extremely sensitive bridges are needed because the element is deposited on a very low thermal conductance material, i.e. glass. Thus, much less energy is required for heating when compared to similar sized elements on alumina substrates. This is illustrated in FIG. 6. It will be seen that the peak temperature is higher and the response time is shorter when the bridge element of this invention is used.

[0047] Although this invention has been disclosed and described in its preferred forms with a certain degree of particularity, it is understood that the present disclosure of the preferred forms is only by way of example and that numerous changes in the details of operation and in the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention as hereinafter claimed. 

I claim:
 1. A bridge-header assembly comprising a header providing first and second electrically conductive sections separated by an electrically insulating section having a thermal conductivity less than about .04 watts/centimeter-K; and a resistive thin film layer on the conductive sections and the insulating section and providing a kerf in the film intersecting the electrically insulating section and separating the film into a first film section in electrical contact with the first conductive section, a second film section in electrical contact with the second conductive section and a third film section bonded to the insulating section providing a resistor through which current between the first and second conductive sections passes.
 2. The assembly of claim 1 further comprising a pyrotechnic charge abutting the resistor.
 3. The assembly of claim 1 wherein the kerf is a laser cut.
 4. The assembly of claim 1 wherein the first film section covers substantially the entire first conductive section and part of the insulating section.
 5. The assembly of claim 4 wherein the second film section covers substantially the entire second conductive section and part of the insulating section.
 6. The assembly of claim 1 wherein the first conductive section comprises a metal pin and the insulating section comprises a sleeve around the pin, the pin comprising a mechanical and electrical connection to an adjacent part.
 7. The assembly of claim 6 wherein the second conductive section comprises a sleeve around the insulating section, the sleeve comprising a mechanical and electrical connection to the adjacent part.
 8. The assembly of claim 7 wherein the header comprises a central axis and the pin lies on the axis.
 9. The assembly of claim 6 wherein the second conductive section comprises a second metal pin offset relative to the first pin, the second pin comprising a mechanical and electrical connection to the adjacent part.
 10. The assembly of claim 1 wherein there is only one resistor and all current between the first section and the second section must pass through the resistor.
 11. The assembly of claim 1 wherein the resistor is of rectilinear shape.
 12. The assembly of claim 1 wherein the kerf provides a pair of parallel segments defining opposite edges of the resistor.
 13. The assembly of claim 12 wherein the kerf provides an end segment perpendicular to and intersecting each of the parallel segments.
 14. The assembly of claim 1 wherein the insulating section has a thermal conductivity less than .03 watts/centimeter-K.
 15. The assembly of claim 14 wherein the insulating section is a glass.
 16. The assembly of claim 1 wherein the header provides a flat surface on which the first and second electrically conductive sections and the electrically insulating section are exposed and the resistive thin film layer is on the flat surface.
 17. A bridge-header assembly comprising a header providing first and second metal pins for connection to a control device, the pins being assembled in a rigid unit having a flat top perpendicular to the pins, the flat top providing a first metal section in electrical communication with the first pin and a second metal section in electrical communication with the second pin and an electrically insulating section of low thermal conductivity electrically separating the first and second metal sections; and a resistive thin film layer on the metal sections and the insulating section and providing a kerf in the film intersecting the electrically insulating section and separating the film into a first film section in electrical contact with the first metal section, a second film section in electrical contact with the second metal section and a third film section bonded to the insulating section providing a resistor through which current between the first and second metal sections passes.
 18. A method of making a bridge-header assembly for a gas generator comprising the steps of providing a header having first and second electrically conductive sections each having at least one pin providing an electrical connection to a control device, the electrically conductive sections being separated by an electrically insulating section of low thermal conductivity; depositing a resistive thin film layer on the conductive sections and the insulating section; and cutting a kerf in the film intersecting the insulating section and separating the film into a first film section in electrical contact with the first conductive section, a second film section in electrical contact with the second conductive section and a third film section bonded to the insulating section providing a resistor of predetermined area through which current between the first and second conductive sections passes.
 19. The method of claim 18 wherein the header comprises a metal pin providing the first conductive section, an insulating sleeve around the pin providing the insulating section and an annular metal member around the insulating sleeve providing the second conductive section and wherein the step of cutting a kerf in the film comprises cutting an arc in the film around and spaced from the pin leaving the third film section between ends of the arc.
 20. The method of claim 18 wherein the header provides a flat surface on which the first and second electrically conductive sections and the electrically insulating section are exposed and the depositing step comprises applying the film to the flat surface.
 21. The method of claim 18 wherein the header comprises a pair of spaced metal pins providing the first and second conductive sections, an insulating sleeve around and separating the pins and providing the insulating section and wherein the step of cutting a kerf in the film comprises cutting at least one segment in the film between the pins.
 22. The method of claim 21 wherein the step of cutting a kerf in the film comprises cutting a pair of segments in the film between the pins, the segments having a pair of parallel adjacent sections providing edges of the resistor. 